1. Field of the Invention
The present invention relates generally to the calibration of rheometers, which are used to characterize materials by measuring the materials' viscosity, elasticity, shear thinning, yield stress, compliance and/or other material properties. More particularly, the invention relates to calibrating the torque output of a rheometer.
2. Background of the Invention
Rheometers, viscometers or viscosimeters are typically used to measure fluid or other properties of materials, such as their viscosity, compliance, and modulus, by rotating, deflecting or oscillating a measuring geometry in a material, either by applying a torque and measuring the resultant velocity or displacement, or by applying a velocity or displacement and measuring the resultant torque. The torque and velocity/displacement are used in conjunction with measuring geometry factors to determine the properties of the material. As used herein, the term “rheometer” shall mean rheometers, viscometers, viscosimeters and similar instruments that are used to measure the properties of fluid or similar (see list below) materials.
The term “measuring object” shall mean an object having any one of several geometries, including, for example, cones, discs, vanes, parallel plates, concentric cylinders and double concentric cylinders. The “materials” may be liquids, oils, dispersions, suspensions, emulsions, adhesives, biological fluids such as blood, polymers, gels, pastes, slurries, melts, resins, powders or mixtures thereof. Such materials shall all be referred to generically as “fluids” herein. More specific examples of materials include asphalt, chocolate, drilling mud, lubricants, oils, greases, photoresists, liquid cements, elastomers, thermoplastics, thermosets and coatings.
As is known to one of ordinary skill in the art, many different geometries may be used for the measuring object in addition to the cylinders, cones, vanes and plates listed above. The measuring objects may be made of; for example, stainless steel, anodized aluminum or titanium. U.S. Pat. No. 5,777,212 to Sekiguchi et al., U.S. Pat. No. 4,878,377 to Abel and U.S. Pat. No. 4,630,468 to Sweet describe various configurations, constructions and applications of rheometers.
The term “calibration” refers to the process of standardizing the rheometer by determining the deviation from an established standard so as to ascertain the proper correction factors for subsequent measurements. Calibration of measuring instruments is vitally important to maintaining the constancy and integrity of the measurements. As known to one of ordinary skill in the art, calibration should be performed whenever possible and to a traceable standard. For rheometers, calibration can be performed to correct measurements of temperature, velocity, displacement, geometry dimensions, and torque, but the present invention is related particularly to the calibration of the torque. The calibration of the torque measurements determines the accuracy and precision of calculated rheological parameters including viscosity, storage modulus, and loss modulus, which are all critically sensitive to the torque value.
Common methods of calibrating viscometers use calibration liquids with known viscosities to correct the measured torque outputs. U.S. Pat. No. 5,509,297 describes a calibration method that plots the viscosity against the measured torque over the range of expected viscosity of the test sample at a specified rotor speed to convert the measured torque into the true viscosity. Another method uses rotating spindles of various sizes depending on the expected viscosity range of the test sample, while taking into account the spindle size in calculating the corrected property value. Calibration methods that use various liquids to correct the viscosity measurements can be significantly and easily influenced by temperature, velocity/displacement, geometry, dimensions, as well as torque in addition to being acutely susceptible to filling errors and contamination.
Other calibration approaches use weights of traceable mass together with either lines and pulleys or strain gauges to calibrate the torque values of rheometers. One proposal to the American Standard of Testing Materials (“ASTM”) for developing a standard for calibration or conformance demonstration for rheometers for the measurement of torque employs a variant form of the line and pulley technique. FIG. 1 is a schematic perspective view of a rotary rheometer 100, showing torque measurement transducer 101, weight of traceable mass 102, line 103 connected to the weight 102, pulley 104, and test fixture 105. The ASTM proposal mounts the test fixture 105 to the bottom of the torque measurement transducer 101 so that the line 103 connected to the weight of traceable mass 102 transmits the force of the mass 102 to the test fixture 105 and the torque measurement transducer 101. The force thus applied produces a measurable torque value, which is then compared to the torque calculated from the applied force. The ratio between the torque output and the applied torque is used to calculate a calibration coefficient to correct subsequent torque measurements.
Calibration methods that use lines, pulleys, or strain gauges tend to be susceptible to both operator errors and systematic errors. For example, the line and pulley method described above requires the operator to make sure that the mass is free hanging without obstruction and that it is not swinging from side to side. Consequently, the need for an experienced operator to perform calibrations increases the costs of operation. In addition, prior art methods are susceptible to various sources for friction that can undermine the accuracy of the calibration and hence the constancy of the instrument. For example, attaching a line to the drive shaft or the torque measurement transducer in rheometers can side-load the shaft bearings, thus creating undesirable interactions with other bearings to produce friction. Even though strain gauges have been used to calibrate torque of rheometers, they are relatively expensive and are therefore not readily available for many rheometer users.
As seen by ASTM's recent interest in the torque calibration of rheometers, there exists a need to develop a simple yet accurate torque calibration technique for rheometers to increase the accuracy of the instrument while reducing sources of friction, costs of equipments, and level of skills required for calibration so that users of rheometers may afford and use their own calibration equipments whenever needed.